c# library for qr code Determining the extent of data loss with DBCC PAGE in Visual C#

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12.4.2 Determining the extent of data loss with DBCC PAGE
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One of the DBCC commands that we haven t spoken of yet is DBCC PAGE, an undocumented (and therefore unsupported) command. If a database page is accessible, DBCC PAGE can be used to inspect its contents. In certain corruption scenarios, this can be very useful in determining the extent of damage. Consider a case where a range of clustered index (data) pages are corrupted for example, pages 98 through 118. By inspecting the pages either side of the corruption range, 97 and 119 in this case, we ll get a much better idea of the extent of damage. Figure 12.6 shows the output of the DBCC PAGE command. Before running the command, we turn on trace flag 3604 to enable the output to be displayed to screen.
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Figure 12.6 Using the undocumented (and unsupported) DBCC PAGE command to inspect the contents of a database page
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As figure 12.6 shows, DBCC PAGE3 will return data from the page that we can use to determine the contents, and thus the potential data loss an important factor in deciding on an appropriate recovery option.
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12.4.3 Recovery options
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To recap, validating the I/O system with SQLIOSIM before production implementation and ensuring page checksums are enabled are crucial steps in avoiding a scenario in which a large amount of data is corrupted before being discovered. SQLIOSIM will
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The four parameters for DBCC PAGE are database name, file number, page number, and print option. Print option 3 includes page header information and row details.
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DBCC validation
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ensure the I/O system is valid and reliable for SQL Server use, and using page checksums in combination with regular event log monitoring helps you identify corrupted pages early, hopefully before the corruption becomes widespread. Of course, neither of these best practices is a guarantee that we ll never have to deal with corrupted data, so knowing the available recovery options is an important step in preparing for the unexpected. Let s walk through the major physical corruption recovery options, beginning with corrupted nonclustered indexes. REBUILDING NONCLUSTERED INDEXES As we saw earlier, if the only corruption found was in nonclustered indexes (index ID 2 and above), then we can use the REPAIR_REBUILD option, or we can simply re-create the index. If the corruption is on a clustered index, we re not as lucky, and the restoration of a recent backup becomes our best option. Fortunately, we re able to restore individual pages. PAGE RESTORE The importance of regular, validated backups can t be overstated, particularly in corruption situations. In some cases, the only way out of a corruption scenario is to restore a backup. The only thing worse than discovering corruption is to then discover the backups are invalid (or don t exist!). Performing regular backups with the WITH CHECKSUM clause, together with regular monitoring for page checksum failures, provides the best chance of detecting corruption early and having reliable backups for recovery. Depending on the scale of corruption, the availability of recent backups may enable the use of the page restore technique. In chapter 10 we looked at online restores, an option available in the Enterprise edition of SQL Server, to restore individual filegroups. Online restores are also available at the page level, enabling us to restore individual corrupted pages. Listing 12.1 shows an example script to restore two pages. Like a filegroup restore, we follow the first restore with one or more transaction log restores, which apply changes made to the pages since the full backup was taken. After the first two restores, we take an additional transaction log backup to ensure all changes to the pages are captured and restored. Finally, we perform all restores with the exception of the last using the WITH NORECOVERY option to enable subsequent restores to occur.
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